Fiber coupler, so called fiber splitter, is an element to separate a light signal from one fiber into multiple fibers. Nowadays, the kinds of the fiber coupler are quite complex because there exists many different demands when being applied in the communication.
When being classified on function, the variety of the fiber coupler can be classified into one by one, one by two and one by N types, etc. And, if being differentiating from the manufacturing method, it can be distinguished into the fused-biconical-tapering and the side-polishing techniques. However, the principles thereof are both based on the evanescent wave coupling method.
In 1981, Kawasaki firstly disclosed a manufacturing method for a biconic tapering single mode fiber coupler, which is still widely adopted now. This method employs a butane-oxygen flame to heat the adjacent un-jacketed fibers and, simultaneously, the fibers are axially elongated and gradually fused while the mode field can thus be getting closer. Since the core mode of the fiber gradually loses the light guiding effect because the core is getting thinner and thinner, the guiding mode thereof will transfer into cladding modes and optical coupling will be occurred between the two fibers. Finally, the fusion will be stopped while a desired splitting ratio of the fibers is achieved through the heating and pulling. Furthermore, the fused region will be sealed in a fillister on a quartz substrate and finally sleeved by a stainless steel cube.
However, in this method, the limitation is that it has a difficulty to raise the temperature of the butane-oxygen flame up to 1500° C. Therefore, when the fibers are heated by the flame, they must simultaneously be mechanically pulled to reduce the fusion point for facilitating the fusion therebetween. At this time, the core of the fiber is so thinned that the effect thereof will be lost, and the mode field will be coupled through expanding the evanescent field to the other fiber. Now, a new core is formed at the fused region which employs the air as a new cladding. Furthermore, the whole fiber fusion region will display a structure similar to a dumbbell.
Nevertheless, because of this dumbbell-like structure, the polarization birefringence effect might be easily induced thereinto. In addition, because the diameter of the fusion region is only about 30 micrometers left, the angle formed as pulling the fiber during fusion should be slowly changed for achieving the adiabatic state of the energy. However, it still can not avoid a drawback of the multi-modes excitation. Besides, because the width of flame is about 5 mm which actually causes the heating region too wide, the pulled fiber might be dropped and deformed due to the gravity. The local air flow and the moisture induced by the flame will also degrade the fiber.
Thus, if an excellent fiber coupler is needed, for example, a narrow band fiber multiplexer/demultiplexer, the elongation length must be longer. However, a long elongation actually will result in an increase of the optical loss and a reduction of the mechanical strength. At the same time, the polarization birefringence effect will accumulate more seriously so as to cause a worse channel isolation. Moreover, hydroxyl ions produced as the flame is combusting will also diffuse into the fiber when heating and pulling thereof so as to cause a seriously loss at the wavelength of around 1.38 μm.
Therefore, this method is not suitable for making the narrow band fiber multiplexer/demultiplexer, the polarization-critical fiber components, E-band component which covers the wavelength of around 1.38 μm, and the components for S-band Raman Amplifier.
Because of the technical disadvantages described above, the applicant keeps on carving unflaggingly to develop a “manufacturing method and apparatus of fiber coupler” through wholehearted experience and research.
Thus, it is an object of the present invention to provide a manufacturing method and apparatus for coupling more than two stacked fibers respectively having an exposed or unexposed evanescent field thereof.
It is another object of the present invention to provide an apparatus employs a movable electric arc for fusing the stacked fibers.
It is a further object of the present invention to provide a manufacturing method and apparatus for a micro-fiber coupler with a super stability.